E-TTL and Manual Flash Settings

Richard

Is there a way to tell after the fact how much flash was actually used in a shot taken in E-TTL mode? I recently acquired an old flash that I can only fire with an optical trigger, so I will need to use my 580-EX in manual mode. I don't have a light meter, so I thought I might be able to reduce the trial and error if I could get a sense of what E-TTL has been doing. I don't see the info in any of the EXIF reading utilities I have, but perhaps there is one out there that can help. :dunno

kdog

Don't think the information is recorded, Richard. But, not sure what good it would do you anyway without knowing a zillion other parameters that would affect the flash output like distance, whether it was bounced and if so, off what, modifiers used, etc, etc. You'll want to keep a notebook anyway if you're serious about reproduceability.

BTW, have you heard of the white towel method of estimating flash exposure? That's where you place a small white towel in the scene and take a few test shots. Keep cranking up the flash power until the white towel just starts to blow out, then back off 1/3 stop. You now have a perfect exposure, although you might still tweak it to taste.

-joel

Richard

kdog wrote:

Don't think the information is recorded, Richard. But, not sure what good it would do you anyway without knowing a zillion other parameters that would affect the flash output like distance, whether it was bounced and if so, off what, modifiers used, etc, etc. You'll want to keep a notebook anyway if you're serious about reproduceability.

-joel

Yeah, I suppose. I'm very lazy and was just looking to see if I could cut down on the trials and errors a bit. Oh well...

Thanks for the towel tip.

ziggy53

Joel's method, using a white towel to measure and protect highlights, would be valuable in many cases. To check general exposure you would normally use a photographic gray card target and expose so that the gray falls in the middle of the histogram. Middle gray in the center of the histogram is the very definition of proper exposure (almost*).

Using manual flash and manual camera settings should give you the most accurate flash exposures possible, assuming that the flash has accurate repeatability in manual mode.

Basically, you can test your camera and flash together and develop your own set of "guide numbers". Flash guide numbers can be used to determine the proper f-stop per distance at a given power output. This should be more accurate than any camera-flash automation because it removes potential influence from things like bright/dark/reflective backgrounds/foregrounds from the equation.

(Don't trust manufacturer guide number ratings, but develop your own through testing.)

As a for instance, if you find that you use an f2 at 10 feet at a power setting of 1/16th on the flash and ISO 100, that indicates a guide number of 20-ft @ ISO 100. If you establish the guide numbers at each power output, you now have a very accurate measure for setting the flash exposure component of any scene with the subject at a given distance using the formula:

GN/Distance=f-number

Adjust as needed for the ISO sensitivity.

http://www.vividlight.com/Articles/1214.htm
http://strobist.blogspot.com/2007/12/guide-number-your-free-flash-meter.html

---

*(The rather complicated explanation about light meters and middle gray measurement)
http://dpanswers.com/tech_kfactor.html

Richard

Thanks for the explanation and links, Ziggy. Looks like I have a lot to learn...but I already knew that.

divamum

kdog wrote:

BTW, have you heard of the white towel method of estimating flash exposure? That's where you place a small white towel in the scene and take a few test shots. Keep cranking up the flash power until the white towel just starts to blow out, then back off 1/3 stop. You now have a perfect exposure, although you might still tweak it to taste.

-joel

I read this once upon a time before I started understanding flash exposure (not that I completely understand it yet, but more than I used to ) and had totally forgotten about it - thanks for mentioning!!

jdorseydesign

My suggestion is to buy David Hobby's Strobist 101 DVDs. they are incredible and well worth it.

http://www.mpex.com/browse.cfm/4,10149.html